1. Field of the Invention
The present invention relates to a discharge and light emitting device in which a discharge gas such as xenon enclosed between electrodes is discharged for emitting light.
2. Description of the Background Art
Various light emitting devices have been proposed and reduced to practice as a light source. One of such conventional devices is a light source used for a contact image sensor (hereinafter as CIS) which reads a content such as diagram.
FIGS. 11 and 12 show an example of such a CIS, CIS 100 including a conventional light source. FIG. 11 is a plan view of CIS 100 disclosed by Japanese Patent Laying-Open No. 4-360458 (Japanese Patent No. 2953595), and FIG. 12 is a cross sectional view of CIS 100 shown in FIG. 11.
As shown in FIGS. 11 and 12, CIS 100 includes an LED (Light Emitting Diode) array 101 as a light source, a casing 102, a sensor IC (integrated Circuit) 103, a rod lens array 104, and a glass plate 105.
A document 106 between a platen 107 and glass plate 105 is irradiated with light by LED array 101, and reflected light is passed through rod lens array 104 to reach sensor IC 103. The reflected light is then converted into an electrical signal by sensor IC 103 and the content of document 106 is read.
The use of LED array 101 as a light source for a contact image sensor as described above is encountered with the following various disadvantages.
When an LED is used as a light source, the necessary light amount of the light source changes depending upon the time required by the image sensor to read one line of information in the case of a line sensor. This means that the signal output I of the sensor has the relation represented as Ixe2x88x9dTxc3x97B relative to the reading speed (reading time T per line) and the brightness B of the light source. Therefore, if reading time T is large (a document is read by a facsimile machine for example at the speed of xcx9c10 ms/line), an output from the sensor is tolerable for use.
Note however that reading time T would be very small for high speed reading at a speed of 0.5 ms/line or less, and therefore sufficient sensor output does not result.
When LED chips are arranged, the optical output of the LED chips has strong directivity, the light amount is much different between forward and diagonally forward directions, and therefore the following problem is encountered. When a light source is manufactured using an arrangement of LED chips, a gap is present between LED chips by the restriction of the mounting pitch, which causes difference in the light amount between the region over the LED chips and the region over the gaps. As a result, a corrugation is generated in the amount of light at the LED mounting pitch in the direction of the arrangement of the LED chips.
Furthermore, because of variation in the LED mounting precision (precision at which the light emitting center of the LEDs is aligned on one line) and the directivity of the light amount as described above, the above corrugation could be larger.
In addition, there is variation in the brightness itself of LED chips, and therefore, using an arrangement of LED chips, the brightness variation is reflected upon the brightness distribution on a line. Therefore, the light amount could not be uniform for the entire illumination length.
When a high brightness is to be achieved, LED chips must be mounted in a high density to increase current contributing to light emission, which however causes the light source to generate heat and hence reduces the useful life of the LED chips.
As a light source for a CIS, a conventional, cylindrical lump such as a hot cathode tube (fluorescent lamp) and a cold cathode tube could be used. In this case, a sufficient amount of light as a light source could be obtained.
However, the inner shape of the CIS must have such a form to receive the cylindrical light source, and therefore the cross sectional shape would be large. Since such a lamp has electrodes at both ends, a lower brightness portion as long as several centimeters called xe2x80x9ccathode dark spacexe2x80x9d is necessarily formed. As a result, the percentage of the region having a stable light amount relative to the entire length of the light source is reduced.
The inventor has eagerly studied and come to conceive the use of a light source which emits light by discharge for the light source of the contact image sensor, and succeeded in the development of a light source of this type. FIG. 1 shows an example of a discharging and light emitting device 1 which can be used as the light source.
As shown in FIG. 1, discharging and light emitting device 1 includes a substrate 2, a transparent substrate 3, an inner electrode 4, an outer electrode 5, a metal bus 6, an insulating layer (dielectric layer) 7, a first fluorescent substance 8, a second fluorescent substance 9, a sealing layer 10, and a discharging space 11.
Substrate 2 and transparent substrate 3 are made for example of glass. Transparent substrate 3 is placed upon substrate 2, and has a wall 3a extending toward substrate 2. Wall 3a is connected to substrate 2 through sealing layer 10 and insulating layer 7. Thus, a discharging space 11 is formed between substrate 2 and transparent substrate 3. A discharge gas such as xenon is enclosed in discharging space 11. Note that sealing layer 10 is made of a glass layer formed for example by melting frit.
Inner electrode 4 is formed on substrate 2, and covered with insulating layer 7. Insulating layer 7 is for example made of a glass layer. First fluorescent substance 8 is formed on insulating layer 7, and second fluorescent substance 9 is formed on the surface of transparent substrate 3.
Outer electrode 5 is for example made of ITO (Indium Tin Oxide) or SnO2, and has transmittancy. Outer electrode 5 formed on the outer surface of transparent substrate 3 forms metal bus 6 on the periphery of outer electrode 5.
In order to allow discharging and light emitting device 1 having the above construction to emit, a prescribed voltage (for example, about 1000V) is applied between inner electrode 4 and outer electrode 5. Thus, a discharge gas is electrolytically dissociated to discharge ultraviolet rays, which are then directed upon first and second fluorescent substances 8 and 9 and these substances emit light.
The inventor has confirmed that the brightness of light thus obtained is higher than the conventional case using the LEDs. The brightness distribution is homogeneous, the useful life of discharging and light emitting device 1 is significantly longer than that of the LEDs. The percentage of the effective illumination length is much increased, which makes it easier to reduce the size in the longitudinal direction. Furthermore, since no toxic substance such as mercury is used, the risk of environmental destruction can be avoided.
While discharging and light emitting device 1 shown in FIG. 1 may provide various, more excellent effects than those of the conventional device as described above, the inventor has further advanced his study to come across the following, new problem to be solved for such discharging and light emitting device 1. The problem will be now described.
FIG. 2 shows a discharging path 12a in discharging space 11 when discharging and light emitting device 1 emits light. Note that the arrow in FIG. 2 represents the direction in which light is emitted.
As shown in FIG. 2, since inner electrode 4 and outer electrode 5 are placed opposing each other, discharging path 12a is positioned vertically to the main surface of each of substrates 2 and 3. The length of discharging path 12a is therefore as short as the shortest distance between substrates 2 and 3.
In a light source using gas discharging, the brightness and light emission efficiency typically increase as a function of the length of the discharging path length. As a result, the short discharging path length as described above could lower the brightness and light emission efficiency in discharging and light emitting device 1.
The present invention is directed to a solution to the above described problem. It is one object of the present invention to provide a discharging and light emitting device providing an increased brightness, a more homogeneous distribution of brightness, prolonged useful life, a higher percentage of effective illumination length and improved light emission efficiency and allowing the longitudinal size to be reduced, and the environmental destruction to be avoided.
A discharging and light emitting device according to the present invention includes first and second substrates, first and second fluorescent substances, and first and second electrodes. The second substrate is placed upon the first substrate to form a discharging space into which a discharging gas is enclosed with the first substrate and has transmittancy. The first and second fluorescent substances are provided in the discharging space. The first electrode is provided on the side of the first substrate and the second electrode is provided on the side of the second substrate. The second electrode is provided shifted from the first electrode such that the first and second electrodes do not overlap.
By thus providing the second electrode shifted from the first electrode, the discharging path can be tilted at a prescribed angle relative to the direction vertical to the main surfaces of the first and second substrates. More specifically, the discharging path can be directed slightly diagonally to the main surfaces of the first and second substrates. As a result, the discharging path length (discharging gap) can be larger than the case shown in FIG. 1, and the brightness and light emission efficiency can be improved. Discharging light emission is caused in the direction connecting the first electrode and the second electrode, and therefore a light emitting region is hardly generated immediately under the electrodes, so that all the emitted light can be taken to the outside. This also contributes to the improved light emission efficiency. Furthermore, if discharged light is taken using discharging, the smaller the density of the current passed at the time of discharging, the greater will be the light emission efficiency. As a result, if the first electrode is provided shifted from the second electrode, strong discharging is generated in the direction connecting the first and second electrodes, while discharging is weak in the other region. Thus, there is a current density distribution, a low current density region is formed, and the light emission efficiency in total can be improved.
The first substrate has the first fluorescent substance and the second substrate has the second fluorescent substance. The first fluorescent substance preferably has a larger thickness than the thickness of the second fluorescent substance. Thus, light can be emitted through the second substrate.
The first electrode is provided on a surface on the discharging space side in the first substrate, the second electrode is provided on an outer surface on the opposite side to the discharging space side in the second substrate. The second electrode is at a ground potential.
Since the second electrode which could be in contact with the outside is set at a ground potential, the risk of electric shock by touching the second electrode can be avoided and the safe operation environment can be secured. When the casing of the discharging and light emitting device is set at a ground potential, the casing and the second electrode do not have to be insulated from one another any longer, which prevents the structure from being complicated and enlarged. Furthermore, the shielding effect against an EMI (radiation noise) from the light taking portion can be provided. The driving frequency for emitting light is in the range from 50 KHz to 100 KHz, and the wavelength is larger than the opening of the light source (discharging and light emitting device), so that the shield effect can be expected for this structure as well.
Preferably, the second substrate has a wall (spacer) extending toward the first substrate, and the second electrode is provided on the inner side than the wall of the second substrate.
If a substance having a large dielectric constant is present between the electrodes other than the discharging gas, a capacitor forms between the substance and the electrodes. The capacitor has a larger capacitance than the discharging gas space. When a voltage to cause light emission is externally applied, the number of charges not contributing to the light emission and charging the capacitor is greater than that of charges for raising the voltage for the discharging space, which lowers the efficiency as a whole (the percentage of the light amount relative to input power). Since the dielectric constant of the wall described above is large, and therefore by providing the second electrode on the inner side than the wall, the wall can be prevented from being provided with voltage, and the efficiency can be prevented from being lowered. Thus, wasteful power consumption can be reduced, and the light emission efficiency can be improved.
The first electrode may be provided on an outer surface positioned on the opposite side to the discharging space side in the first substrate, while the second electrode may be provided on a surface on the discharging space side in the second substrate. In this case, the first electrode is set at a ground potential.
An insulating layer covering the second electrode and having transmittancy is preferably provided. Thus, light can be let out through the insulating layer.
The above insulating layer may be provided with an opening to reach the second substrate, and the second fluorescent substance may be formed on the surface of the second substrate positioned in the opening.
When light is emitted to the outside from the insulating layer side, the above opening permits light to be passed through the opening for emission to the outside. Thus, the brightness can be increased as compared to the case of emitting light through the insulating layer to the outside. The transparency of the insulating layer does not have to be improved in this case.
A wall extending toward the second substrate may be formed on the side of the first substrate. In this case, the first electrode is provided on the inner side of the wall of the first substrate. Also in this case, the light emission efficiency can be improved.
The first electrode is formed into a flat plate (strip) shape, and the second electrode is formed into an annular shape. In this case, discharging is caused on both sides of the first electrode, and the brightness and light emission efficiency can be further improved. In addition, discharging light emission is generated in the region surrounded by the annular second electrode, a light emitting region is less likely to be formed immediately under the electrodes, and therefore all the emitted light can be let out. Also in this case, a low current density region can be positively produced, which improves the light emission efficiency in total. If one side of the second electrode being disconnected, it will not be defective and the margin relative to the disconnection of electrodes can be improved. Furthermore, the outlet of light can be clearly defined by the second electrode.
The discharging and light emitting device according to the present invention is particularly useful as a light source for a contact image sensor.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.